The present invention relates to a scroll fluid machine suitable for use in an air compressor, a vacuum pump, etc. by way of example. More particularly, the present invention relates to a scroll fluid machine provided with a variable crank for varying the orbiting radius of an orbiting scroll member.
In general, a scroll fluid machine has a casing and a fixed scroll member provided in the casing and having a spiral wrap portion standing on an end plate. A driving shaft is rotatably provided in the casing. An orbiting scroll member is orbitably provided on the distal end of the driving shaft. The orbiting scroll member has a spiral wrap portion standing on an end plate. The wrap portion overlaps the wrap portion of the fixed scroll member to define a plurality of compression chambers.
These days, there is known a scroll fluid machine in which a fitting portion is provided at the distal end of the driving shaft, while a boss portion is provided on the orbiting scroll member, and an orbiting radius varying mechanism is provided between the fitting portion of the driving shaft and the boss portion of the orbiting scroll member. The orbiting radius varying mechanism is fitted to the fitting portion and the boss portion to vary the orbiting radius of the orbiting scroll member [for example, see Japanese Patent Application Unexamined Publication (KOKAI) No. 09-144674 (1997)].
In the scroll fluid machine using such an orbiting radius varying mechanism, the wrap portion of the orbiting scroll member and the wrap portion of the fixed scroll member are always in contact with each other at a plurality of points. Assuming that the two wrap portions contact each other at the inner peripheral surface of the wrap portion of the orbiting scroll member on one side of the center of orbiting motion of the orbiting scroll member along one diameter, the two wrap portions contact each other at the outer peripheral surface of the wrap portion of the orbiting scroll member on the other side of the center along the same diameter. Accordingly, when moving in one direction, the orbiting scroll member is subjected to frictional forces in the opposite direction to the direction of movement at a plurality of contact points. At this time, the frictional forces occurring on one side of the center of the orbiting scroll member and those occurring on the other side of the center act on the orbiting scroll member so as to urge it to rotate in opposite directions to each other.
The orbiting scroll member is prevented from rotating. In actuality, however, there is backlash between the orbiting scroll member and the rotation preventing mechanism. Therefore, when the tendency of the orbiting scroll member-orbiting mechanism to urge the orbiting scroll member to rotate is overcome by the tendency of the total sum of the above frictional forces to urge the orbiting scroll member to rotate in the opposite direction, the orbiting scroll member rotates slightly in the opposite direction. Accordingly, the structure of the prior art causes vibration and noise unfavorably.
In view of the above-described problems with the prior art, an object of the present invention is to provide a scroll fluid machine in which the orbiting scroll member is constantly urged in a direction in which rotational torque acts and so it is allowed to orbit smoothly.